When an artificial material is implanted in a damaged bone of a living body, the material is generally surrounded by membranes of collagen fibers and thus isolated from neighboring bones. There have been known some artificial materials, which are not isolated by such fibrous membranes and connect to bones in a living body strongly and naturally. Examples of such artificial materials include Na2O—CaO—SiO2—P2O5-based bioglasses, sintered hydroxyapatites (Ca10(PO4)6(OH)2), and crystallized glasses A-W containing apatite microcrystals and β-wollastonite microcrystals (CaO, SiO2). These materials are referred to as bioactive ceramics, and some of them have been put into practical use as important bone-restoring materials.
Among the bioactive ceramics, sintered hydroxyapatites are well known for high biocompatibility and have been most widely used in clinical applications as bone supplements, replacement bone, artificial vertebrae, artificial joints, bioactive coating materials of artificial dental roots, etc. Also methods of producing the sintered hydroxyapatites have been widely studied. With demand for more biocompatible artificial bones increasing in recent years, however, it is desired to develop a bone-restoring apatite material containing a magnesium ion or a carbonate ion like a bone in a living body.
The carbonated apatites are so lower in decomposition temperature than the hydroxyapatites that they are sintered at relatively low temperatures to provide carbonated apatite ceramics. JP 2000-72572 A discloses a molded implant produced by plastically working a sintered apatite body, and a method for producing the molded implant, which comprises the steps of sintering an apatite at 900° C. or lower, charging the sintered apatite into a predetermined mold, and plastically working the sintered apatite at 300 to 780° C. Because the sintering temperature is low in this method, a carbonated or fluorinated apatite with a low decomposition temperature can be used to produce the implant having high biocompatibility. Further, JP 3308355 B discloses a method for producing a sintered carbonated apatite, which comprises sintering at 600 to 850° C. However, these implants mainly comprise the apatites without other crystal phases, thereby having low mechanical strength.
Effective methods for producing dense sintered bodies at low temperatures include liquid phase sintering methods using glasses as sintering aids. By using the liquid phase sintering methods, sintered apatites with improved mechanical strength can be produced such that a bioactive glass is softened around main crystals of an apatite such as a carbonated apatite in a bone-restoring ceramic material and generates crystals between the main crystals. Conventionally, non-bioactive glasses are used as the sintering aids of the sintered hydroxyapatite body. However, because the non-bioactive glasses have high glass transition temperatures and/or crystallization temperatures, they cannot generate preferable crystals by sintering at temperatures lower than the decomposition temperatures of the carbonated apatites. JP 2934090 B discloses a biocompatible implant having a bending strength of 40 MPa or more, which is produced by adding a calcium phosphate-based glass frit to a hydroxyapatite and by burning the resultant mixture. However, the biocompatible implant has a high porosity of 5 to 55%, and this method cannot produce a dense biocompatible implant. Proposed in Japanese Patent Application No. 2002-206319 corresponding to U.S. Ser. No. 10/618,687 are a bioactive glass low in glass transition temperature and/or crystallization temperature, and a sintered calcium phosphate body using the bioactive glass, which has high biocompatibility, large mechanical strength, and excellent sinterability. However, the bioactive glass cannot be sufficiently softened at the decomposition temperature of the carbonated apatite or lower, and thus sintered carbonated apatite bodies using the bioactive glass are not sufficient in mechanical strength.